Technical Field
The present invention relates to the use of recombinant LAG-3 or derivatives thereof in order to boost a monocyte-mediated immune response. It enables an increase in monocyte numbers in blood. It finds many applications in particular in the development of novel therapeutic agents in cancer immunotherapy.
State of the Art
In the description which follows, the references between brackets [ ] refer to the attached reference list. The lymphocyte activation gene 3 (hlag-3) expressed in human CD4+ and CD8+ activated T cells as well as in activated NK cells encodes a 503 amino acids type I membrane protein (LAG-3) with four extracellular immunoglobulin superfamily (IgSF) domains [1]. A murine lymphocyte activation gene 3 (mlag-3) was cloned and approximatively 70% of homology was found with hlag-3, with the same proline rich motif in the intracytoplasmic tail.
LAG-3 (CD223), described as being a natural high affinity ligand for MHC class II, is known to induce maturation of monocyte-derived dendritic cells in vitro, and is used as an immunotherapy adjuvant to induce CD4 T helper type 1 responses and CD8 T cell responses in vivo [2]. Further information regarding LAG-3 and its use as an immunostimulant may be found in TRIEBEL et al. [1], TRIEBEL et al. [3], and HUARD et al. [4]. Some forms of soluble LAG-3 can bind to MHC class II molecules and can induce dendritic cells to mature and migrate to secondary lymphoid organs where they can prime naïve CD4-helper and CD8-cytotoxic T cells leading to tumour rejection [5]. More recently a recombinant soluble human LAG-3Ig fusion protein (IMP321) was shown to activate a large range of effector cells in both innate and acquired immune responses, for example inducing monocytes-macrophages to secrete cytokines/chemokines [6].
Monocytes are produced by the bone marrow from haematopoietic stem cell precursors called monoblasts. They constitute between three to eight percent of the leukocytes in the blood. Monocytes circulate in the bloodstream for about 24 hours (half-life of 8 hours) and then typically move into tissues throughout the body. In the tissues, monocytes mature into macrophages, epitheloid cells or antigen-presenting cells (APCs, for example dendritic cells). Monocytes are responsible for phagocytosis (ingestion) of foreign substances in the body. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the pathogen directly via pattern-recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody, termed antibody-dependent cell-mediated cytotoxicity (ADCC). Due to their secretion and phagocytosis properties, monocytes-macrophages occur in aspecific and specific immune response.
The study of membrane markers allows the identification of monocyte populations, mature or not, dystrophic or not. The molecules present on monocyte membranes, mature or not, are almost always non specific but correspond to the following activities:                receptor for the Fc fragment of IgG (CD16, CD32, CD64),        receptor for the Fc fragment of IgE (CD23),        receptor for complement fractions (CD11b, CD21/CD35),        leukocyte adhesion proteins (CD11a, CD11c),        protein facilitating binding to LPS of Gram-bacteria (CD14),        protein with tyrosine phosphatase activity (CD45).        